The use of electric or electromagnetic fields to promote healing, particularly the healing of fractured bones, has been investigated since the early 19th Century. See Spadaro, "Bioelectric Stimulation of Bone Formation: Methods, Models and Mechanisms," Journal of Bioelectricity, 1(1), 99-128 (1982). In early research, direct current techniques were used by applying electrodes to the skin or via the use of implanted electrodes into the bone. More recently, mechanisms which encourage growth have been investigated which involve the use of electromagnetic fields to induce voltage and current effects within the tissue. These techniques have been particularly useful in non-healing or "nonunion" fractures by inducing bones to heal which will not heal naturally.
An example of a technique for the use of electromagnetic radiation to promote bone growth is Ryaby et al., U.S. Pat. No. 4,266,532, issued May 12, 1981. Ryaby et al. showed effective techniques for promoting bone growth. However, the techniques disclosed in Ryaby et al. required the use of power applied from a standard wall socket. Electromagnetic therapy is only useful so long as the patient uses it. Being tethered to the wall is a sufficient annoyance such that many patients will not follow the electrotherapy regimen prescribed by their doctors.
Recognizing this problem, a great deal of work has occurred to try to develop mechanisms whereby portable electrotherapy may be provided. Examples of such techniques are shown in Talish, et al., U.S. Pat. No. 4,574,809, issued Mar. 11, 1986; Christianson et al., U.S. Pat. No. 4,432,361, issued Feb. 21, 1984, Castel, U.S. Pat. No. 4,587,957, issued May 13, 1986; and co-pending application Ser. No. 899,674 assigned to the Assignee of the present patent application. Christianson et al. requires the use of invasive electrical probes into the bone. It is desirable to avoid invasive techniques if possible because of the possibility of infection. Talish et al., Castel and Serial No. 899,674 use magnetic transducers designed to provide a uniform electromagnetic field throughout the treatment area. Talish et al. uses a Helmholtz coil as does Ryaby et al. Ser. No. 899,674 uses a solenoid surrounding the injured limb to generate the magnetic field. Castel uses a polarized magnetic field. These types of transducers generate a great deal of electromagnetic radiation outside of the damaged body portion and waste a great deal of energy inside the damaged body portion by radiating areas where radiation is unnecessary. For example, the tibia and scaphoid bones are very close to the surface of the skin. Use of the techniques shown in the above references causes radiation to be generated throughout the entire limb even though the damaged portion is close to the surface of the skin. This problem is even more striking in the case of bones in the trunk of the body such as vertebrae and ribs. Therefore, the technique for applying electromagnetic fields in a manner which concentrates their application on the desired area while minimizing wasted electromagnetic energy is desirable.
The benefits of electromagnetic stimulation of damaged or diseased tissue are being further developed and are widely accepted by today's scientific community. Examples of studies which show the benefits of electromagnetic stimulation to soft tissue as well as bone are Black, "Electrical Stimulation of Hard and Soft Tissues in Animal Models, " Clinics in Plastic Surgery, Vol. 12, No. 2, pages 243-257 April, 1985 and Frank et al., "A Review of Electromagnetically Enhanced Soft Tissue Healing," IEEE Engineering in Medicine and Biology Magazine, pages 211-32 December, 1983. In addition, diseased rather than broken bones may benefit from electromagnetic therapy. For example, see Brighton, U.S. Pat. No. 4,467,808, and Bassett et al., "Treatment of Osteonecrosis of the Hip with Specific Pulsed Electromagnetic Fields (PEMFs): A Preliminary Clinical Report," Bone Circulation, Chapter 56 pages 343-357, edited by Arlet, Ficat and Hungerford (1984).